1. Field of the invention
This invention relates to an image sensor which converts images from the image surface of documents, etc., into electrical signals using an array of photoelectric conversion elements, more particularly to a charge storage type of contact image sensor.
2. Background of the prior art
In order to reduce in size of image reading equipment which is used in facsimile machines, etc., contact image sensors are used. These contact image sensors are ones which read the image data at a size of almost 1:1, and signal reading, particularly in charge storage contact image sensors, is carried out by a voltage reading method.
The basic configuration of this type of image sensor is shown in FIG. 9.
In this FIG., P is a charge storage type of photoelectric conversion element made from photodiode D which puts out an electric charge according to the interelectrode capacitance C.sub.0 and the amount of incident light, and is normally arranged in an array.
One end of each of these photoelectric conversion elements P is connected to drive power source E, and the other ends are each connected to switching element S of integrated circuit I through conductive strips L. Switching elements S are driven in order by shift register SR, and the charge signal storaged by photoelectric conversion element P is output.
A switching element S becomes ON in sequence, and during the time until reading one line is complete and it becomes ON once more, the charge produced by photoelectric conversion element accumulates in inter-terminal capacitance C.sub.0, and when the switching element S which corresponds to that accumulated charge becomes ON, it is read out. This charge which is read out passes through a detecting circuit and is output as read output.
However, in this type of existing image sensor, photoelectric conversion element P and integrated circuit I are connected by conductive strips L formed on the substrate as dedscribed above, but the lengths of these conductive strips L on the integrated circuit mounting are not standardized, and since the wiring capacitance of each of the conductive strips is non-uniform, there is the problem that distortions are produced in the output signal.
The conductive strips have two wiring capacitances, ground capacity C.sub.1, and wiring stray capacitances C.sub.2 (stray capacitances of the conductive strips defining wiring patterns), and if the remaining capacitance produced by integrated circuit I is taken as C.sub.3, and the charge which is storaged in photoelectric conversion element P is taken as Q, then with a voltage reading method, the photoelectric conversion element output signal at the end of the wiring pattern is given by formula (1), and the photoelectric conversion element output signal for other sections is given by formula (2). EQU Q/(C.sub.0 +C.sub.1 +C.sub.2 +C.sub.3) (1) EQU Q/(C.sub.0 +C.sub.1 +2C.sub.2 +C.sub.3) (2)
Consequently, when the conductive strips L become long or high density, variations in wiring capacitance (C.sub.1 +C.sub.2) increase, there is an accompanying increase in variations in the output signal, so that for example, as shown in FIG. 10, when marks, such as black mark 1a and lighter mark 1b, are read by image sensor 2 the output signal from each photoelectric conversion element P is not standardized, and as shown in FIG. 11, distortions are produced in the output. For this reason, if the output signal is normally read as "1" or "0", a threshold level SL is taken. However, in color sensing devices, etc., since detection of gray scale lightness is necessary, then, as shown in FIG. 12, for example, 2 threshold levels have to be taken, and because of this, output signals need to be corrected as shown in FIG. 13 using output correction circuits. However, adding this type of correction circuit gives rise to the problems of complicating the image sensor construction and increasing product costs.
As a means of correcting this sort of output variation, as shown in FIG. 14, a method has been proposed of evening out variations in wiring capacitance (C.sub.1 +C.sub.2) and adjusting ground capacity C.sub.1 by changing the conductive strips L of integrated circuit I so that their width decreases in proportion with their length. In this drawing T is a photoelectric conversion element connection terminal, and W is bonding wire.
However, even though this method corrects conductive strip ground capacity C.sub.1 by having the normal photoelectric conversion element array pitch equal, there is the problem that because the spaces between adjacent conductive strips are not equal, the wiring stray capacitance C.sub.2, which is one of the stray capacitances, is non-uniform. The inventors also attempted to consider a method of correcting non-uniformity of stray capacitances in conductive strips L, produced by differences in wiring length, by changing interterminal capacitance C.sub.0 of photoelectric conversion element P. This is not shown in the diagram. However, since in comparison with conductive strip L stray capacitances C.sub.1, C.sub.2, for a low resolution image sensor of 1-4 dots/mm, inter-terminal capacitance C.sub.0 is large, then correction changes greatly according to fluctuations in the photoetching process, that is to say, according to changes in photoetching conditions, etc. For this reason, the sum of this capacitance and the stray capacitances could not be standardized, the degree of improvement in image sensor output current uniformity was samll, and the method could not be put into practical use. In addition, for a high resolution image sensor of 8 dots/mm or higher, since stray capacitances C.sub.1, C.sub.2 in conductive strips L become larger than inter-terminal capacitance C.sub.0, in addition to correction only having a small effort, conversely, when the length (in the sub-scanning direction) of the terminal which determines photoelectric conversion element inter-terminal capacitance C.sub.0 is substantially changed to increase C.sub.0, then the sub-scanning direction reading position varies between photoelectric conversion elements, and because of this, there are problems in application with regard to reading accuracy. Furthermore, in addition to correcting the wiring capacitances, the size of the conductive strips increases, causing an increase in the size of the image sensor.